1. Field of the Invention
The present invention relates to a spark-ignition engine controller, and more particularly to a device for controlling the combustion condition in each cylinder of a multi-cylinder engine in order to improve fuel economy and reduce emissions.
2. Description of the Related Art
Conventional technology for improving fuel economy in a spark-ignition engine by performing combustion with the air-fuel ratio of the air-fuel mixture in each cylinder set to a lean air-fuel ratio which is larger thin the stoichiometric air-fuel ratio is well known. It is also known that by providing a fuel injector for injecting fuel directly into a combustion chamber and performing stratified charge combustion by injecting fuel from the fuel injector in the compression stroke when the engine is being operated in a low-speed low-load range or the like, ultra lean combustion can be realized (see Japanese Unexamined Patent Application Publication H10-274085 for example).
In this type of engine, when a typical three-way catalyst (a catalyst with a good purification performance in respect of HC, CO, and NOx in the vicinity of the stoichiometric air-fuel ratio) alone is used as an exhaust gas purifying catalyst, a sufficient purifying performance cannot be obtained in respect of NOx during a lean operation, and hence, as described in Japanese Unexamined Patent Application Publication H10-29836, a lean NOx catalyst is provided to adsorb the NOx in an oxygen-rich atmosphere and then release and reduce the NOx in a reduced oxygen concentration atmosphere. When the amount of NOx adsorbed by the lean NOx catalyst increases during a lean operation in cases where such a lean NOx catalyst is used, additional fuel is injected in the expansion stroke as well as that used in the main combustion, as described in the aforementioned publication, for example. Hence the air-fuel ratio of the exhaust gas becomes rich and CO is generated, thereby precipitating the release and reduction of the NOx.
In such conventional engines which perform lean operations, the aforementioned lean NOx catalyst must be provided in the exhaust passage to maintain the NOx purification ability during a lean operation, which is not cost-effective. Also, in order to maintain the purification ability of the lean NOx catalyst, the air-fuel ratio must be made temporarily rich due to the additional fuel supply and the like for releasing and reducing the NOx when the amount of adsorbed NOx increases, as described above. If the fuel in use has a high sulfur content, the lean NOx catalyst must be heat processed and regeneration processed by supplying reducing agent or the like in order to prevent sulfur poisoning of the catalyst, and in so doing the effect of an improvement in fuel economy decreases. Furthermore, if the air-fuel ratio of the mixture becomes lean to or above a certain extent, the combustion speed becomes too slow and combustion near the final combustion phase does not contribute to the work. Hence improvements in fuel economy through lean stratified charge combustion are limited.
Compression ignition has been researched as an alternative method of improving fuel economy. In compression ignition, the temperature and pressure inside the combustion chamber are raised in the final phase of the compression stroke, similarly to a diesel engine, whereby the fuel is caused to self-ignite. When such compression ignition is performed, the entire content of the combustion chamber burns at once even when the air-fuel ratio is ultra lean or a large amount of EGR is introduced, and thus slow combustion which does not contribute to the work is avoided, thereby improving fuel economy.
In a typical spark-ignition gasoline engine, however, forced ignition is required for combustion, and thus the temperature and pressure inside the combustion chamber in the vicinity of compression top dead center cannot be raised to an extent at which compression ignition occurs. In order to cause compression ignition, specific measures must be taken to greatly raise the temperature or pressure inside the combustion chamber, but in the past it has been difficult to raise the temperature or pressure inside the combustion chamber to an extent at which compression ignition occurs in the partial load range at which fuel economy improvement is required while avoiding irregular combustion caused by heavy knocking in the high-load range, or in other words spontaneous ignition of the air-fuel mixture before the flame propagates through the combustion chamber.
Hence in order to achieve a great improvement in fuel economy by using lean combustion and compression ignition in combination, the present applicant has filed technology relating to a spark-ignition engine controller according to which, in a partial load range of the engine, a two-cylinder connection is created between a pair of cylinders having an overlapping exhaust stroke and intake stroke such that burned gas discharged from a preceding cylinder in the exhaust stroke is introduced as is into a following cylinder in the intake stroke via an intercylinder gas channel. In the preceding cylinder, the air-fuel ratio is set to a lean air-fuel ratio that is greater than the stoichiometric air-fuel ratio such that combustion is performed by forced ignition, whereas in the following cylinder fuel is supplied to the lean air-fuel ratio burned gas introduced from the preceding cylinder such that combustion is performed by compression ignition (Japanese Patent Application 2002-185242).